Transcript Powerpoint

IJSO Training Course
Phase II
Module: Energy
Time allocation: 10 hours
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Objectives
 Introduce the concepts of heat, work, and internal
energy.
 Discuss the main energy transformations that take
place in a power station.
 Discuss the advantage and disadvantages of producing
electrical energy using various energy resources, e.g.,
fossil fuel, nuclear fission, and some renewable
energy resources, e.g., solar energy, hydro-electric
power, etc.
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1. Work, Energy and Power
 When a force moves a body in the direction along
which it acts, work is done on the body.
 Energy is the ability to do work, work is the
process of converting energy.
 To calculate the work done by a moving force:
Work done = force x distance moved in direction of force
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 That is,
W = F•s
 the unit of work or energy is the Joule (J).
1 J is the quantity of work done when a
force of 1 N moves a body 1m, along its
own line of action.
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
Although force and displacement are both vectors,
work (or energy) is a scalar quantity.

Examples:
Vectors: force, displacement, velocity
Scalar: temperature, energy, speed
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
Exercises:
1. How much work is done when a force of 4 kN moves
a block 1000cm in the direction of the force?
(40000J)
2. Find the work done in raising 85 kg of water through
a vertical distance of 4m.
(3332 J)
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
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If the direction of the displacement is not the same
as the direction of the force, we use the component
of the force which is parallel to the displacement.
For example, when a body is caused to accelerate
down an inclined plane by the force of gravity:

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Therefore, the more general equation to calculate
W = F•s
cosθ q is the
work done is
, where
angle between the force and the displacement.
2.


Kinetic Energy
A body in motion possesses kinetic energy.
The kinetic energy possessed by a body depends on its
mass and its speed. Consider a body of mass m and
speed v, its kinetic energy is given by :
K.E. = ½ (mv2)
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
3. Potential Energy
Examples:
1. A mass in a gravitational field possesses gravitational
potential energy.
2. A stretched spring possesses elastic potential energy.
3. A charged body in an electric field possesses
electrical potential energy.
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Gravitational Potential Energy

Consider a ball of mass m being lifted from A to B,
the increase in the gravitational potential energy is
given by :
Gravitational P.E. = m g h

where g = 9.8 ms-2 is the acceleration due to gravity.
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
Note: The equation tells
- only the change in G.P.E. of the body, and
-is valid for a short distance near the earth’s surface,
i.e., the gravity g is constant.
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
Exercises:
1. What is the change in potential energy if a body of 7
kg on the ground (a) is lifted to 120 m. (b) is placed
at the bottom of a vertical mine shaft 120m deep.
(a) 8.2 kJ, (b) -8.2 k J
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2. A car of mass 500 kg traveling at 20m/s has its
speed reduced to 5m/s by a constant breaking force
over a distance of 80m.
(a) The cars initial kinetic energy
(b) The final kinetic energy
(c) The breaking force
(a) 100 kJ, (b) 6.25 kJ, (c) 1172 N
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Elastic Potential Energy

Consider a spring being stretched an extension is x,
by Hooke’s law, the restoring force is given by :
F = - kx
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
The elastic potential energy stored in the spring is
given by the area under the curve:

or it equals:
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Elastic P.E. = ½ (kx2)
4.

Power
Power is work done per second (or energy
converted per second).
P = W/t


unit of power is the Joule per second, Js-1. 1 Js-1 is
called 1 Watt (1W).
Combining this equation with the definition of work
we have:
P = Fv
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
Exercises:
1. A constant force of 3kN pulls a crate along a level
floor a distance of 15 m in 40s. What is the power
used?
1125W
2. A hoist operated by an electric motor has a mass of
600 kg. It raises a load of 300 kg vertically at a steady
speed of 0.5 m/s. Frictional resistance can be taken to
be constant at 900 N. What is the power required?
4860W
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3. A car of mass 1000 kg has an engine with power
output of 45 kW. It can achieve a maximum speed of
110 km/h along the level.
(a) What is the resistance to motion?
(b) If the maximum power and the resistance
remained the same what would be the maximum
speed the car could achieve up an incline of 1 in 40
along the slope?
(a) 1473 N, (b) 26.2 m/s
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5. The Law of Conservation of Energy
 Energy can never be destroyed, it is just
converted from one form to another.
 Examples of Energy Conversions:
1. A battery converts chemical energy into
electrical energy.
2. An electric motor lifting an elevator
converts electrical energy into gravitational
potential energy.
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3. When a moving vehicle uses its brakes to
stop, the kinetic energy of the vehicle is
converted to heat in the brakes.
4. A nuclear reactor converts nuclear
energy into heat.
5. Sound is caused by the vibration of air
molecules.
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
Example:
Now the ball is at rest at B
and is falling from a height
h.
-At B, the ball has
gravitational potential
energy only, i.e., E = mgh;
-At A, the elephant has
kinetic energy only, i.e.,
E = ½ mv2
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By the law of conservation of energy,
m g h of
=½
Therefore, the speed
the elephant at A is
2
mv
given by :
v = √(2gh)
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
Exercises:
1. Describe the energy transformation for the
following processes.
(a) A block is released down a smooth inclined
plane.
(b) A bullet strikes a wooden block and is
embedded inside.
(c) Two blocks resting on a smooth horizontal
surface held against a compressed, light spring are
released.
(d) Streams of water fall from the top to the bottom
of a waterfall.
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2. A cyclist and his bicycle has a mass of
90 kg. After 200 m he reaches the top of
a hill, with slope 1 in 40 measured along
the slope, at a speed of 3 m/s. He then
free wheels the 200m to the bottom of the
hill where his speed has increased to
7m/s. How much energy has he lost on
the hill?
2610J
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6.

Machines
Examples:
Lever (left) and pulley (right)
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
Efficiency of a machine:
-Work input into a machine:

Work input into a machine = Effort x distance moved by
effort
work input = E x dE
-Work output by a machine:
Work output by a machine = Load x distance moved by load
work output = L x dL
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- Efficiency of a machine:
Efficiency (h) of a machine = (work input/ work output) x
100%
h = [(L x dL)/(E x dE)]x 100%
- Mechanical advantage of a machine:
M.A. = Load / Effort = L/E
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-Velocity ratio of a machine:
V.R. = distance moved by effort / distance moved by
-Efficiency of a machine
can be rewritten as:
load =dE/dL
h = (M.A. / V. R.) x
100%
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
Exercises:
1. A machine has an efficiency of 70%.
The effort moves 5 m in lifting a load of
25 kg through 0.5 m. Find
(a) the useful output of the machine,
(b) the total input of the machine,
(c) the effort required.
(a) 122.5 J, (b) 175 J, (c) 35 N
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

2. A car of mass 800 kg, running at 55 km h-1, consumes
5.25 liters of petrol per hour. The time taken for the car
to decelerate from 70 km h-1 to 40 km h-1 when engaged
in neutral gear is found to be 14 s.
(a) Calculate the deceleration of the car when its speed is
55 km h-1.
(b) What is the resistance acting on the car at this speed?
(c) (i) What is the work output by the engine in 1 hour
when the car is moving at km/hr?
(ii) If 4.55 litres of petrol can release 1.7 x 108J of
energy, find the efficiency of the engine of the car.
(a) 0.595ms-2, (b) 476.2 N, (c) (i) 2.62 x 107J, (ii) 15.4%
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7.

Some kinds of machines
The levers
Scissors (left) and nutcrackers (right)
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The pulleys
V.R. = ______V.R. = ______V.R. = ______
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The screw jack
V.R. = __________
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8. Fossil fuels as a major source of
energy

Forms of energy can be classified as
1. Non-renewable energy: those which are
exhaustive, i.e., fossil fuels (石化燃料) and
nuclear energy
2. Renewable energy: those which will not
be exhaustive, e.g., solar power, water
power, wind power, etc.
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
Fossil fuels is commonly used today for
electricity generation, and come in 3
principal forms: Coal (煤) , Natural Gas (天然
氣) and Crude Oil (原油).
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Formation of Coal


Organic material made up primarily of
carbon, varying amounts of hydrogen,
oxygen, nitrogen and sulphur.
It was formed millions of years ago in the
Carboniferous Period 石炭紀時期 (~360 to
286 million years ago). As the trees and
plants died, they sank to the bottom of the
swamps of oceans, forming layers of a
spongy material call peat (泥煤).
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

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Peat is a brownish material that looks
like wood. Although it can be burnt as a
fuel, it contains a lot of water, and
therefore is very smoky when burnt.
As the peat became buried beneath more
sand, clay and other plant matter, the
pressure and temperature increased and
the water was squeezed out of it. Over
many years the materials became
compacted the carbon content increased,
it turned into carbon-rich coal.
Formation of Oil


Fossil fuels: liquid forms - crude oil, gaseous
forms - natural gas.
Between 10 to 160 million years ago, marine
plants and animals died and sank to the
bottom, they were rapidly covered in mud,
sand, and other mineral deposits. This rapid
burial prevented immediate decay.
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

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In a lack-of-oxygen environment, the
organisms were slowly decayed into
carbon-rich compounds. These
compounds mixed with surrounding
sediments and formed source rock.
As more layers were deposited on top of
one another, pressure and heat acting on
the source rock compressed the organic
material into oil.

As the movement of the earth, oil travels
into rocks that have larger spaces, or pores,
to hold it. Limestone and sandstone are two
types of rocks with large pores, and they are
called porous rocks. These reservoir rocks
were trapped between impermeable cap rock,
which can hold oil within the ground for
many years.
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Crude oil


It has various
components, which
do not have the same
boiling points.
At oil refineries, crude
oil is split into
various types of
products by heating.
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It then made into many different
products - the clothes, the toothbrush,
the plastic bottle, the plastic pen. Almost
all plastic comes originally from crude oil.
The products include gasoline, diesel
fuel, aviation or jet fuel, home heating oil,
oil for ships and oil to burn in power
plants to make electricity.
Natural gas



Mostly made up of a gas called methane
(CH4), which is made up of 1 carbon and 4
hydrogen atoms.
Lighter than air, highly flammable (易燃)
Colourless and no odor. It is mixed with a
chemical that gives a strong rotten-egg odor
before being sent to storage tanks.
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Safety Note


If you smell that rotten egg smell in your
house, get out of the house quickly.
Don't turn on any lights or other electrical
devices. A spark from a light switch can
ignite the gas easily.
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Saving Fossil Fuels


Fossil fuels take millions of years to make.
We are using up the fuels that were made
more than 300 million years ago before the
time of the dinosaurs.
So, it's better not to waste fossil fuels. They
are not renewable. We can save fossil fuels
by conserving energy.
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9.Alternative energy source: Nuclear
power
Nuclear energy



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Nuclear energy is not renewable.
Matter and energy can't be created nor
destroyed, but they can be changed in
form. Matter can be changed into energy.
That is the
mass-energy equation:
.
E=mc
2
This equation says mass and energy are
equivalent. In which, E [energy] equals m
[mass] times c2 , where c is the speed of
light.

When a neutron bombards on a heavy nucleus,
e.g., uranium 235, it splits into several smaller
fragments. 2 or 3 neutrons are emitted
spontaneously. This process is called nuclear
fission. The sum of the masses of these fragments
is less than the original mass. This 'missing' mass
(~ 0.1 %) has been converted into energy
according to the mass-energy equation.
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

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Nuclear chain reaction refers to a process
in which neutrons released in fission
produce an additional fission in further
nucleus. If each neutron releases one
more neutron, then the number of
fissions doubles each generation. In that
case, in 10 generations there are 1,024
fissions.
The rate of the reaction can be controlled
(nuclear power) or uncontrolled (nuclear
weapons).
Controlled nuclear reaction


Nuclear power stations work much the same
way as fossil fuel-burning stations, except
that a nuclear fission inside a nuclear
reactor makes the heat instead.
Modern reactors use enriched uranium-235
as fuel. Natural uranium is only 0.7%
uranium-235; the rest is uranium-238.
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

Neutrons smash into the nucleus of the
uranium atoms, which split the atom and
release energy in the form of heat.
Pressurized water is pumped through the
reactor to take the heat away, and the hot
gas then heats water to make steam. The
steam drives turbines which drive
generators.
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
Control rods (made of boron or graphite) are
to absorb extra neutrons in order to control
the rate of nuclear reaction.
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

If the reactor gets too hot (nuclear reaction
is too fast), the control rods are lowered into
the reactor to absorb neutrons. Hence, the
reaction rate is decreased.
If the reaction is slow down, the control
rods are raised. More neutrons crash into
uranium atoms and hence more energy is
generated.
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Advantage of using nuclear power




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It is relatively not expensive to build a
nuclear power plant.
Produces huge amounts of energy from
small amounts of fuel.
Does not produce smoke or carbon
dioxide, so it does not contribute to the
greenhouse effect.
Small amounts of waste are produced.
Disadvantage of using nuclear power



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Radioactive waste is produced. It must
be sealed up and buried for many years
to allow the radioactivity to die away.
Terrorism may increase in scale. Reactors
could be a target being attacked with
bombs.
Much effort (e.g., money) is paid for safety
measures. If it goes wrong, a nuclear
accident can be a major disaster.
10. Renewable energy sources
Solar power




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Solar energy is free, renewable and
available to everywhere in the world.
No waste or pollution is produced, few
environmental problems are created.
Exempt from rising energy prices.
Good for remote locations.
However,
 Produces a small energy output per
surface area of solar cell.
 Not good for large-scale production. It
requires thousands of mirrors or cells
that take up a large area of land.
 It’s relatively expensive to build solar
power stations.
 Does not work at night or in a cloudy day.
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Hydro-electric power




The efficiency is high and the running costs
are low.
No waste or pollution produced.
More reliable than other renewable energy
resources such as wind or solar power.
Water can be stored above the dam ready to
cope with peaks in demand.
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However,
 Very expensive to build a dam, but many
dams are also used for flood control, so
building costs can be shared.
 Building a large dam will flood a very
large area upstream, causing problems
for animals that used to live there.
 Water quality and quantity downstream
can be affected, which can have an
impact on plant life.
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Wind power



Cheap and clean, produces no waste or
greenhouse gases.
Provide electricity to remote areas.
Wind farms can be tourist attractions.
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However,
 The wind is not always predictable - some
days have no wind.
 Suitable areas for wind farms are often
near the coast, where land is expensive.
 Noisy. A wind generator makes a
constant, low, "swooshing" noise day and
night.
— End —
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